An internal combustion engine comprises a plurality of cylinders, each of which is connected to an intake manifold by means of at least one corresponding intake valve and to an exhaust manifold by means of at least one corresponding exhaust valve. The intake manifold is connected to a filtering device for receiving fresh air (i.e. air from the outside environment) and is regulated by a butterfly valve, while the exhaust manifold is connected to an exhaust system, which has the function of releasing into the atmosphere gases produced by the internal combustion limiting both the noise and the content of polluting substances.
In an internal combustion engine working according to the “Diesel” cycle (i.e. fed with diesel fuel or the like), to improve the combustion within the cylinders the use of a main recirculation circuit has been suggested, which is regulated by a recirculation valve and connects the exhaust manifold to the intake manifold to introduce into the intake manifold a certain amount of the exhaust gases present in the exhaust manifold and produced by the combustion which previously occurred in the cylinders. In order to not excessively penalise the volumetric efficiency of the engine, before being introduced into the intake manifold, the recirculated exhaust gases may be cooled by a heat exchanger coupled to the main recirculation conduit.
An example of an internal combustion engine provided with a main recirculation circuit of the type described above is described in patent application EP1674698A2 or in patent application DE4240239A1.
In order to reduce the production of NOx during combustion within the cylinders, the use of a secondary recirculation circuit has recently been suggested, which takes a certain amount of the exhaust gases present in the exhaust system downstream of a gas treatment device consisting of a catalyser and an anti-particulate filter. It must be noted that the exhaust gases recirculated by the secondary recirculation circuit display a lower average temperature with respect to the exhaust gases recirculated by the main recirculation system and thus allow to obtain a better cooling of the combustion chambers in the cylinders with a consequent greater reduction of the production of NOx; furthermore, the exhaust gases recirculated by the secondary recirculation system display a much lower content of particulate with respect to the exhaust gases recirculated by the main recirculation circuit and thus have less negative effects on the combustion and on the lubrication within the cylinders.
An example of an internal combustion engine provided with a main recirculation circuit and a secondary recirculation circuit is described in patent application EP1621755A2.
Experimental tests have shown that in an internal combustion engine provided with a main recirculation circuit and a secondary recirculation circuit, the impeller of the turbocharger which compresses the intake air is at high risk of breakage during the first months of operation of the engine. In order to avoid this drawback, the use of reinforced impellers has been suggested, which however display both higher manufacturing costs, and greater mass (i.e. greater mechanical inertia and thus a longer response delay).